Sp1-mediated downregulation of P2X4 receptor gene transcription in endothelial cells exposed to shear stress

Endothelial purinoceptors play an important role in vascular responses to extracellular adenine nucleotides and hemodynamic forces. Here we report that P2X4 purinoceptor expression in human umbilical vein endothelial cells is transcriptionally downregulated by fluid shear stress. When human umbilical vein endothelial cells were subjected to a laminar shear stress of 15 dyn/cm(2), P2X4 mRNA levels began to decrease within 1 h and further decreased with time, reaching 60% at 24 h. Functional analysis of the 1.9-kb P2X4 5'-promoter indicated that a 131-bp segment (-112 to +19 bp relative to the transcription start site) containing a consensus binding site for the Sp1 transcription factor was critical for the shear stress responsiveness. Mutations of the Sp1 site decreased the basal level of transcription and abolished the response of the P2X4 promoter to shear stress. Electrophoretic mobility shift assays showed a marked decrease in binding of Sp1 to the Sp1 consensus element in shear-stressed cells, suggesting that Sp1 mediates the shear stress-induced downregulation of P2X4 gene transcription.

Fluid shear stress activates Ca(2+) influx into human endothelial cells via P2X4 purinoceptors

Ca(2+) signaling plays an important role in endothelial cell (EC) responses to shear stress generated by blood flow. Our previous studies demonstrated that bovine fetal aortic ECs showed a shear stress-dependent Ca(2+) influx when exposed to flow in the presence of extracellular ATP. However, the molecular mechanisms of this process, including the ion channels responsible for the Ca(2+) response, have not been clarified. Here, we demonstrate that P2X4 purinoceptors, a subtype of ATP-operated cation channels, are involved in the shear stress-mediated Ca(2+) influx. Human umbilical vein ECs loaded with the Ca(2+) indicator Indo-1/AM were exposed to laminar flow of Hanks' balanced salt solution at various concentrations of ATP, and changes in [Ca(2+)](i) were monitored with confocal laser scanning microscopy. A stepwise increase in shear stress elicited a corresponding stepwise increase in [Ca(2+)](i) at 250 nmol/L ATP. The shear stress-dependent increase in [Ca(2+)](i) was not affected by phospholipase C inhibitor (U-73122) but disappeared after the chelation of extracellular Ca(2+) with EGTA, indicating that the Ca(2+) increase was due to Ca(2+) influx. Antisense oligonucleotides designed to knockout P2X4 expression abolished the shear stress-dependent Ca(2+) influx seen at 250 nmol/L ATP in human umbilical vein ECs. Human embryonic kidney 293 cells showed no Ca(2+) response to flow at 2 micromol/L ATP, but when transfected with P2X4 cDNA, they began to express P2X4 purinoceptors and to show shear stress-dependent Ca(2+) influx. P2X4 purinoceptors may have a "shear-transducer" property through which shear stress is perceived directly or indirectly and transmitted into the cell interior via Ca(2+) signaling.

P2X(4) receptors mediate ATP-induced calcium influx in human vascular endothelial cells

ATP induces Ca(2+) influx across the cell membrane and activates release from intracellular Ca(2+) pools in vascular endothelial cells (ECs). Ca(2+) signaling leads to the modification of a variety of EC functions, including the production of vasoactive substances such as nitric oxide and prostacyclin. However, the molecular mechanisms for ATP-induced Ca(2+) influx in ECs have not been thoroughly clarified. Here we demonstrate evidence that a P2X(4) receptor for an ATP-gated cation channel is predominantly expressed in human ECs and is involved in the ATP-induced Ca(2+) influx. Northern blot analysis distinctly showed the expression of P2X(4) mRNA in human ECs cultured from the umbilical vein, aorta, pulmonary artery, and skin microvessels. Competitive PCR revealed that P2X(4) mRNA expression was much higher in ECs than was the expression of other subtypes, including P2X(1), P2X(3), P2X(5), and P2X(7). Treatment of ECs with antisense oligonucleotides designed to target the P2X(4) receptor decreased the P2X(4) mRNA and protein levels to approximately 25% of control levels and markedly prevented the ATP-induced Ca(2+) influx.

The effect of flow on the neutrophil-mediated Ca2+ responses in human vascular endothelial cells stimulated by endotoxin

Leukocyte-vascular endothelial cell (EC) interactions which promote inflammatory and immune reactions involve bidirectional signaling between two cell types. We investigated the effects of flow on neutrophil-mediated changes in endothelial intracellular Ca2+ levels ([Ca2+]i). Cultured human umbilical vein ECs stimulated by endotoxin were labeled with Fura-2 and exposed to fluid flow with neutrophils. The individual changes in [Ca2+]i were monitored. The application of flow with neutrophils to stimulated ECs led to an increase in [Ca2+]i although either flow without neutrophils or neutrophils without flow rarely induced a rise in [Ca2+]i. Furthermore, flow application with neutrophils to unstimulated ECs also rarely promoted a rise in [Ca2+]i. These findings suggest that the flow might thus induce or enhance the inflammatory process by the induction of Ca2+ signaling in endotoxin-stimulated endothelium facing neutrophils in the blood flow.

Fluid shear stress transcriptionally induces lectin-like oxidized LDL receptor-1 in vascular endothelial cells

Fluid shear stress has been shown to modulate various endothelial functions, including gene expression. In this study, we examined the effect of fluid shear stress on the expression of lectin-like oxidized LDL receptor-1 (LOX-1), a novel receptor for atherogenic oxidized LDL in cultured bovine aortic endothelial cells (BAECs). Exposure of BAECs to the physiological range of shear stress (1 to 15 dyne/cm2) upregulated LOX-1 protein and mRNA in a time-dependent fashion. LOX-1 mRNA levels peaked at 4 hours, and LOX-1 protein levels peaked at 8 hours. Inhibition of de novo RNA synthesis by actinomycin D totally abolished shear stress-induced LOX-1 mRNA expression. Furthermore, nuclear runoff assay showed that shear stress directly stimulates transcription of the LOX-1 gene. Chelation of intracellular Ca2+ with quin 2-AM completely reduced shear stress-induced LOX-1 mRNA expression; furthermore, the treatment of BAECs with ionomycin upregulated LOX-1 mRNA levels in a dose-dependent manner. Taken together, physiological levels of fluid shear stress can regulate LOX-1 expression by a mechanism dependent on intracellular Ca2+ mobilization. Inducible expression of LOX-1 by fluid mechanics may play a role in localized expression of LOX-1 and atherosclerotic lesion formation in vivo.

Physiologic shear stress suppresses endothelin-converting enzyme-1 expression in vascular endothelial cells

Shear stress dilates blood vessels and exerts an antiproliferative effect on vascular walls. These effects are ascribed to shear stress-induced, endothelium-derived vasoactive substances. Endothelin-converting enzymes (ECEs), the enzymes that convert big endothelin-1 (ET-1) to ET-1, have recently been isolated and the corresponding proteins have been termed ECE-1 and ECE-2. Furthermore, two isoforms of human ECE-1 have been demonstrated and termed ECE-1 alpha and ECE-1 beta. In this study, to elucidate the role of ECE-1 under shear stress we examined the effect of physiologic shear stress on the mRNA expression of ECE-1 and ET-1 in cultured bovine carotid artery endothelial cells (BAECs) and human umbilical veins (HUVECs), and also ECE-1 alpha mRNA expression in HUVECs. ECE-1 mRNA expression was significantly downregulated by shear stress in 24 h, both in BAECs and HUVECs, in a shear stress intensity-dependent manner. The expression of ECE-1 alpha mRNA was also attenuated by shear stress in HUVECs. ET-1 mRNA expression showed a concordant decrease with ECE-1 mRNA expression. These results suggest that shear stress-induced gene regulation of ET-1 and ECE-1 mRNA expression can contribute to the decrease of ET-1 peptide level by shear stress.

Endothelial Ca2+ waves preferentially originate at specific loci in caveolin-rich cell edges

Stimulation of endothelial cells (ECs) with ATP evoked an increase in intracellular Ca2+ concentration ([Ca2+]i). In a single bovine aortic EC, the [Ca2+]i rise started at a specific peripheral locus and propagated throughout the entire cell as a Ca2+ wave. The initiation locus was constant upon repeated stimulation with ATP or other agonists (bradykinin and thrombin). The Ca2+ wave was unaffected by the removal of extracellular Ca2+, demonstrating its dependence on intracellular Ca2+ release. Microinjection of heparin into the cell inhibited the ATP-induced Ca2+ responses, indicating that the Ca2+ wave is at least partly mediated by the inositol 1,4, 5-trisphosphate receptor. Immunofluorescence staining revealed that caveolin, a marker protein for caveolae, is distributed heterogeneously in the cell and that Ca2+ waves preferentially originate at caveolin-rich cell edges. In contrast to caveolin, internalized transferrin and subunits of the clathrin-associated adaptor complexes such as adaptor protein-1 and -2 were diffusely distributed. Disruption of microtubules by Colcemid led to redistribution of caveolin away from the edges into the perinuclear center of the cell, and the ATP-induced [Ca2+]i increase was initiated on the rim of the centralized caveolin. Thus, caveolae may be involved in the initiation of ATP-induced Ca2+ waves in ECs.

Fluid shear stress increases the production of granulocyte-macrophage colony-stimulating factor by endothelial cells via mRNA stabilization

To investigate whether the production of colony-stimulating factors (CSFs) by vascular endothelial cells is regulated by hemodynamic force, we exposed cultured human umbilical vein endothelial cells (HUVECs) to controlled levels of shear stress in a flow-loading apparatus and examined changes in the production of CSFs at both the protein and mRNA level. Exposure of HUVECs to a shear stress of 15 and 25 dyne/cm2 markedly increased the release of granulocyte-macrophage CSF (GM-CSF) detected by ELISA to 5.0 and 9.5 times, respectively, the amount released by the static controls at 24 hours, but it had no significant influence on the release of granulocyte CSF or macrophage CSF. The results of reverse transcriptase-polymerase chain reaction demonstrated that GM-CSF mRNA began to increase as early as 2 hours after initiation of 15 dyne/cm2 shear stress and continued to increase with time, reaching a peak of about four times the control levels at 24 hours. This increase in GM-CSF mRNA levels in response to shear stress depended on protein synthesis, because it was blocked by cycloheximide. Neither nuclear run-on assay or luciferase assay using a reporter gene containing GM-CSF gene promoter showed any significant change in transcription of the GM-CSF gene even after 24-hour exposure to a shear stress of 15 dyne/cm2. Actinomycin D chase experiments using a competitive polymerase chain reaction showed that shear stress extended the half-life of GM-CSF mRNA from approximately 23 to 42 minutes in HUVECs. These findings suggest that fluid shear stress increases the production of GM-CSF in HUVECs via mRNA stabilization.

Cloning of cDNAs encoding G protein-coupled receptor expressed in human endothelial cells exposed to fluid shear stress

A cDNA library of human umbilical vein endothelial cells exposed to fluid shear stress was constructed to search for functional endothelial genes expressed under flow conditions, and cDNAs encoding members of the G protein-coupled receptor (GPCR) family were cloned by a polymerase chain reaction (PCR) method using degenerate oligonucleotide primers. One of the two GPCR clones obtained was edg-1, and the other clone is a novel gene named FEG-1 that encodes a 375-amino acid protein similar to the receptors for both angiotensin II and chemokines. Reverse transcriptase-PCR showed that the FEG-1 and edg-1 mRNA levels in endothelial cells increased markedly in response to fluid flow. This suggests that FEG-1 and edg-1 may be receptor genes that play important roles in the regulation of endothelial function under physiological blood flow conditions.

Negative transcriptional regulation of the VCAM-1 gene by fluid shear stress in murine endothelial cells

To explore the mechanism of shear stress-induced downregulation of vascular cell adhesion molecule 1 (VCAM-1) expression in murine endothelial cells (ECs), we examined the effect of shear stress on VCAM-1 gene transcription and assessed the cis-acting elements involved in this phenomenon. VCAM-1 mRNA expression was downregulated at the transcriptional level as defined by nuclear run-on assay and transient transfection of VCAM-1 promoter-luciferase gene constructs. The luciferase assay on the VCAM-1 deletion mutants revealed that the cis-acting element is contained between -694 and -329 bp upstream from the transcription initiation site. Gel shift assay using overlapping oligonucleotide probes of this region showed that oligonucleotides containing a double AP-1 consensus sequence (TGACTCA) formed distinct complexes with nuclear proteins extracted from shear-stressed cells. Mutation of either one or both of two AP-1 consensus sequences completely abolished the ability of the promoter to respond to shear stress. These results suggest that fluid shear stress downregulates the transcription of the VCAM-1 gene via an upstream cis-element, a double AP-1 consensus sequence, in murine lymph node venule ECs.

Shear stress augments expression of C-type natriuretic peptide and adrenomedullin

Shear stress is known to dilate blood vessels and exert antiproliferative effects on vascular walls: these effects have been ascribed to shear stress-induced upregulation of endothelium-derived vasoactive substances, mainly nitric oxide and prostacyclin. We have demonstrated the significance of C-type natriuretic peptide (CNP) as a novel endothelium-derived relaxing peptide (EDRP) that shares a cGMP pathway with nitric oxide. Adrenomedullin is a recently isolated EDRP that elevates intracellular cAMP as prostacyclin does. To elucidate the possible role of these EDRPs under shear stress, we examined the effect of physiological shear stress on CNP mRNA expression in endothelial cells derived from the human umbilical vein (HUVECs), bovine aorta (BAECs), and murine lymph nodes (MLECs) as well as adrenomedullin mRNA expression in HUVECs. CNP mRNA was stimulated prominently in HUVECs under shear stress of 15 dyne/cm2 in a time-dependent manner (4 hours, sixfold increase compared with that in the static condition; 24 hours, 30-fold increase). Similar results were obtained in BAECs (4 hours, twofold increase; 24 hours, threefold increase) and MLECs (4 hours, threefold increase; 24 hours, 10-fold increase). Augmentation of CNP mRNA expression that was dependent on shear stress intensity was also observed (5 dyne/cm2, 2.5-fold increase of static; 15 dyne/cm2, 4.5-fold increase). Increased CNP secretion was also confirmed by the specific radioimmunoassay for CNP. Adrenomedullin mRNA expression in HUVECs increased under shear stress of 15 dyne/cm2 in a time-dependent manner (4 hours, 1.2-fold increase of static: 24 hours, threefold increase) and shear stress intensity-dependent manner (15 dyne/cm2, threefold increase compared with that at 5 dyne/cm2). These results suggest that the coordinated augmentation of mRNA expression of these novel EDRPs may constitute shear stress-dependent vasodilator and antiproliferative effects.

Differential display and cloning of shear stress-responsive messenger RNAs in human endothelial cells

To investigate the effect of shear stress on endothelial gene expression, we performed differential display of mRNAs from cultured human umbilical vein endothelial cells either incubated under static conditions or exposed to shear stress (15 dynes/cm2) for 6 h in a flow-chamber. Around 4% of the total number of mRNAs detected were either up- or down-regulated by shear stress. DNA sequencing of some of these shear stress-responsive mRNAs revealed homology of several clones to known gene sequences and many other clones for unknown genes. Known genes, including those for human laminin B1 chain, H(+)-ATP synthase coupling factor 6, lysyl oxidase, myosin light chain kinase, and interleukin-8 receptor, were upregulated by shear stress, while the gene encoding NADH dehydrogenase was down-regulated. The present results suggest that shear stress can change the expression of numerous genes in endothelial cells, far more than reported to date, and that mRNA differential display is quite useful for cloning known and unknown shear stress-responsive genes.

Contribution of sustained Ca2+ elevation for nitric oxide production in endothelial cells and subsequent modulation of Ca2+ transient in vascular smooth muscle cells in coculture

To elucidate the intracellular Ca2+ (Ca2+i ) transient responsible for nitric oxide (NO) production in endothelial cells (ECs) and the subsequent Ca2+i reduction in vascular smooth muscle cells (VSMCs), we administrated four agonists with different Ca2+i-mobilizing mechanisms for both cells in iso- or coculture. We monitored the Ca2+i of both cells by two-dimensional fura-2 imaging, simultaneously measuring NO production as NO2-. The order of potency of the agonists in terms of the peak Ca2+i in ECs was bradykinin (100 nM) > ATP (10 microM) > ionomycin (50 nM) > thapsigargin (1 microM). In contrast, the order in reference to both the extent of Ca2+i reduction in cocultured VSMCs and the elevation in NO production over the level of basal release in ECs completely matched and was ranked as thapsigargin > ionomycin > ATP > bradykinin. Treatment by NG-monomethyl-L-arginine monoacetate but not indomethacin or glybenclamide restored the Ca2+i response in cocultured VSMCs to the isoculture level. In ECs, when the Ca2+ influx was blocked by Ni2+ or by chelating extracellular Ca2+, all four agonists markedly decreased NO production, the half decay time of the Ca2+i degenerating phase, and the area under the Ca2+i curve. The amount of produced NO hyperbolically correlated to the half decay time and the area under the Ca2+i curve but not to the Ca2+i peak level. Thus, the sustained elevation of Ca2+i in ECs, mainly a result of Ca2+ influx, determines the active NO production and subsequent Ca2+i reduction in adjacent VSMCs. Furthermore, L-arginine but not D-arginine or L-lysine at high dose (5 mM) without agonist enhanced the NO production, weakly reduced the Ca2+i in ECs, and markedly decreased the Ca2+i in VSMCs, demonstrating the autocrine and paracrine effects of NO (Shin, W. S., Sasaki, T., Kato, M., Hara, K., Seko, A., Yang, W. D., Shimamoto, N., Sugimoto, T., and Toyo-oka, T. (1992) J. Biol. Chem. 267, 20377-20382).

Flow stimulates ICAM-1 expression time and shear stress dependently in cultured human endothelial cells

Human umbilical vein endothelial cells were subjected to controlled levels of shear stress in a flow-loading apparatus, and changes in the expression of intercellular adhesion molecule-1 (ICAM-1) and vascular cell adhesion molecule 1 (VCAM-1) were measured by flow cytometry. Application of shear stress (15 dynes/cm2) increased the cell surface expression of ICAM-1 2.7 times the control level 4 hr after the onset of flow, while it caused no change in VCAM-1 expression. The increase of ICAM-1 expression by shear stress was time- and force-dependent and reversible. Flow loading using perfusates with different viscosity revealed that the increase in ICAM-1 was shear-stress- rather than shear-rate-dependent. Reverse transcriptase/polymerase chain reaction analysis showed upregulation of ICAM-1 mRNA levels by shear stress, whose time course closely paralleled that of the cell surface protein. These results suggest that shear stress generated by blood flow acts as a regulator of cell adhesion molecule expression on vascular endothelial cells.

Down-regulation of vascular adhesion molecule-1 by fluid shear stress in cultured mouse endothelial cells

This study was undertaken to determine whether blood flow modulates the adhesive property of vascular endothelial cells to lymphocytes and, if it does, what adhesion molecules are involved. Cultured mouse endothelial cells were exposed to medium flow in a parallel plate chamber, and binding assay using fluorescence-labeled lymphocytes was carried out. The adhesion rate of endothelial cells to lymphocytes, which was high in the static control state, decreased when exposed to shear stress (1.5 dynes/cm2) for 6 h. The treatment of static endothelial cells with a monoclonal antibody of vascular cell adhesion molecule-1 (VCAM-1) depressed the adhesion rate to the same extent as that caused by flow, while monoclonal antibodies of CD44 and intercellular adhesion molecule-1 had no effect on it. Flow cytometric analysis revealed that the application of flow decreased markedly the amount of VCAM-1 expressed on the cell surface. A reverse transcriptase-polymerase chain reaction of mRNA showed that flow depressed VCAM-1 mRNA levels. These results suggest that blood flow can modulate the adhesive property of endothelial cells to lymphocytes via affecting the surface expression of adhesion molecules, e.g., down-regulation of VCAM-1.

Fluid shear stress increases the expression of thrombomodulin by cultured human endothelial cells

Endothelial cells (ECs) cultured from human umbilical vein were exposed to medium flow in a flow-loading chamber, and changes in thrombomodulin (TM) expression were examined by flow cytometry and enzyme linked immunosorbent assay with monoclonal antibody. The expression of TM antigen was increased time- and shear stress-dependently by flow, and when exposed to a shear stress of 15 dynes/cm2 for 24 hr, it increased to approximately 200% of the stationary control level. Reverse transcriptase-polymerase chain reaction showed that TM mRNA levels in ECs also increased in response to flow. TM mRNA began to increase one hour after the application of shear stress of 15 dynes/cm2 and reached a maximum (approximately 330% of stationary control) after eight hours. These results, demonstrating an up-regulating effect of flow on TM expression in ECs, suggest that shear stress may be an important modulator of intravascular blood coagulation.

Exogenous nitric oxide inhibits proliferation of cultured vascular endothelial cells

Cultured bovine fatal aortic endothelial cells (BAECs) were stimulated with nitric oxide (NO)-releasing vasodilators and NO gas-saturated solution, and changes in the cell proliferation were examined. Sodium nitroprusside (SNP) and nitroglycerin (NTG) shifted the growth curve downward, and inhibited 3H-thymidine incorporation by the ECs in a dose-dependent manner. Application of NO solution also reduced 3H-thymidine incorporation. SNP, NTG and NO solution increased the intracellular cGMP in BAECs. A cGMP analog, 8-bromo-cGMP, inhibited 3H-thymidine incorporation, and a guanylate cyclase inhibitor, methylene blue, almost completely blocked the inhibitory effect of SNP and NTG on 3H-thymidine incorporation. These findings suggest that exogenous NO inhibits EC proliferation, and that intracellular cGMP is involved in the inhibitory effect of NO.

Shear stress inhibits adhesion of cultured mouse endothelial cells to lymphocytes by downregulating VCAM-1 expression

Monolayers of endothelial cells (EC) cultured from mouse lymph nodes were exposed to controlled levels of shear stress (0-7.1 dyn/cm2) in a parallel plate flow chamber, and binding between the flow-loaded EC and mouse lymph node-derived lymphocytes was assayed. A large number of lymphocytes adhered to the stationary control EC, but in EC exposed to a shear stress of 1.5 dyn/cm2 for 6 h, the adhesion decreased to 68.8 +/- 12.8% (SD; n = 19) of control (n = 29, P < 0.001). The decrease in adhesion induced by flow loading was time and shear stress dependent and reversible. Treatment of stationary EC with a monoclonal antibody (MAb) to vascular cell adhesion molecule-1 (VCAM-1) reduced the adhesion to 70.6 +/- 11.5% (n = 19) of control (P < 0.001), whereas MAb to CD44 and to intercellular adhesion molecule-1 had no effect on it. Flow cytometric analysis revealed that the amount of VCAM-1 expressed on the cell surface was decreased to 48.5 +/- 15.8% (n = 6) of control by flow loading (P < 0.001). Flow loading experiments using two perfusates with different viscosities demonstrated that the decrease in VCAM-1 expression due to flow was shear stress rather than shear rate dependent. The detection of mRNA by reverse transcriptase-polymerase chain reaction showed that VCAM-1 mRNA levels were markedly depressed in EC exposed to flow loading.(ABSTRACT TRUNCATED AT 250 WORDS)

Laminar flow stimulates ATP- and shear stress-dependent nitric oxide production in cultured bovine endothelial cells

Based on the fact that nitric oxide (NO) production is associated with changes in intracellular cGMP levels and is selectively inhibited by N omega-methyl L-arginine (L-NME), we investigated the shear stress dependency of NO production in endothelial cells (ECs) from its cGMP responses to various shear stress loads. Cultured fetal bovine aortic ECs treated with a phosphodiesterase inhibitor, isobutylmethylxanthine (IBMX; 1 mM), were exposed to a laminar flow of Krebs buffer solution for 5 minutes in a parallel-plate flow chamber and examined for changes in intracellular cGMP levels by radioimmunoassay using an [125I] cGMP kit. Application of flow increased the cGMP levels. The increase was significant in the presence of extracellular ATP (1 microM)(control, 286.1 +/- 43.6; flow, 506.5 +/- 44.9 fmol/10(7) cells; p < 0.001), but not in its absence (control, 256.6 +/- 60.6; flow, 301.5 +/- 91.4 fmol/10(7) cells; N.S.). The cGMP levels increased significantly as the magnitude of shear stress applied increased. Treatment of ECs with a specific inhibitor of NO production, L-NMA (200 microM), completely inhibited the flow-induced increase in cGMP, and L-arginine reversed the L-NMA-induced inhibition, indicating that the increase in cGMP was due to NO produced by the flow. The flow-induced increase in NO production was markedly suppressed when extracellular Ca++ was chelated by adding EGTA to the perfusate. These findings suggest that flow stimulates NO production to increase cGMP levels shear stress-dependently in ECs and that extracellular Ca++ and ATP modulate the effects of flow.

Intracellular calcium response to directly applied mechanical shearing force in cultured vascular endothelial cells

We studied the responses of cultured endothelial cells to mechanical shearing force directly applied to those cells in vitro to determine changes in the concentration of intracellular calcium ion (Ca++), one of the factors that transfers information within the cell. Cultured bovine fetal aortic endothelial cells containing the Ca++ fluorescence indicator, Fura-2, were rubbed with a latex balloon in a specially designed system, and changes in the fluorescence of Fura-2 caused by this shear stimulation were determined by photometric fluorescence microscopy. Immediately after shear stimulation, the concentration of Ca++ in the cells was increased and reached a peak (511 +/- 165 nM, n = 12) within 15 seconds after stimulation. After the peak, the concentration was gradually restored to the resting level (55 +/- 17 nM, n = 12). The magnitude of the Ca++ response was dependent on the intensity of the shear force applied. Analysis of fluorescence images of Fura-2 revealed that the cells showed this Ca++ reaction without being injured or desquamated, although there were slight differences in the degree and duration of reaction among cells. This reaction appeared even when the cells were placed in the air with no contact with the fluid. This result suggests that neither the fluid flow associated with the balloon movement nor chemical substances in the fluid are involved in the reaction, but that pure physical force alone is responsible for the Ca++ reaction. Further, it suggests that endothelial cells have the ability to perceive such physical stimulation as shear force and to transfer this information to the interior of the cell via changes in the intracellular Ca++ concentration.

[Release of endothelium-derived relaxing factor (EDRF) from cultured vascular endothelial cells depends on extra- and intracellular Ca2+ concentrations]

The objective of this study was to elucidate the role of extracellular and intracellular Ca2+ in EDRF release from vascular endothelial cells. Cultured fetal bovine aortic endothelial cells were stimulated by adenosine triphosphate (ATP), calcium ionophore (A23187), bradykinin (BKN) and acetylcholine (ACh). The amount of EDRF released was determined on the basis of the relaxation of rabbit aortic ring and the cGMP content of cultured smooth muscle cells derived from fetal bovine aorta. Changes in intracellular Ca2+ concentration ([Ca2+]i) were measured by a photometric fluorescence microscopy using Fura-2. ATP and A23187 induced dose-dependent increases in both [Ca2+]i and EDRF release. BKN increased both [Ca2+]i and EDRF release upon initial exposure (10(-8) M), but there were no further changes at higher concentrations because of desensitization. ACh induced no significant changes in either [Ca2+]i or EDRF release. There was a statistically significant correlation between agonist-induced changes in [Ca2+]i and the amount of EDRF released (p < 0.01). Removal of extracellular Ca2+ eliminated the continuous elevation induced by agonists of both [Ca2+]i and the amount of EDRF. The agonist-induced EDRF release was inhibited by L-NMA (N omega-methyl L-arginine). The EDRF release reduced by L-NMA was restored by adding L-arginine. Thus, the EDRF released from endothelial cells is thought to be an L-arginine-derived nitric oxide (NO) or an NO-related substances. These findings suggest that [Ca2+]i is closely linked to the amount of EDRF released, and that extracellular Ca2+ is essential for EDRF release because its influx is involved in the continuous elevation of [Ca2+]i.

[Role of hemodynamic factors in atherogenesis]

Since atherosclerotic lesions are apt to occur at specified regions in the blood vessels, hemodynamic factors, such as shear stress generated by blood flow, have been considered to play a role in atherogenesis. Atheroma, which is characterized by the localized accumulation of lipid and the proliferation of smooth muscle cells in the intima, appears at branching or curving sites of blood vessels, where both geometrical shape and blood flow change suddenly. In such sites, both stagnant and turbulent blood flow can occur and the direction and intensity of shear stress, acting on the vascular wall, changes transiently. Recent studies using cultured endothelial cells (EC) and flow-loading apparati have demonstrated that shear stress modulates various EC functions. Shear stress alters EC macromolecular permeability and affects the production of growth factors, by the EC, which stimulate smooth muscle cell proliferation. Shear stress also exerts an influence on EC turnover, which might be involved in the transport of low-density lipoproteins via leaky junctions. Furthermore, shear stress modulates the interaction between leukocytes and EC by changing the expression or functions of adhesion molecules. It is very likely that changes in EC functions induced by shear stress are involved in atherogenesis, but direct evidence demonstrating the role of shear stress in the initiation of atherosclerotic disease processes is still lacking. Further studies are needed to clarify the role of hemodynamic factors in atherogenesis.

The effect of flow on the expression of vascular adhesion molecule-1 by cultured mouse endothelial cells

Adherence of leukocytes to vascular endothelial cells (ECs) is known to be sensitive both to blood flow and adhesive proteins on EC surface. To elucidate the effect of blood flow on the surface expression of adhesive proteins, cultured ECs derived from mouse lymph nodes were exposed to different levels of wall shear stress in a flow-loading chamber, and changes in the expression of vascular adhesion molecule-1 (VCAM-1) and CD44 were evaluated by immunostaining with monoclonal antibodies and flow cytometry. Both proteins were expressed on non-activated cultured ECs. When exposed to flow with shear stress of 1.5 dynes/cm2 for 24 hr, VCAM-1 nearly disappeared on fluorescence micrographs, while CD44 showed no change. Flow cytometric analysis showed that the mean channel fluorescence of VCAM-1 was decreased about 75% by application of flow for 24 hr (p < 0.001), but that of CD44 remained unchanged. VCAM-1 expression began to decrease around 1 hr after the initiation of flow and became markedly reduced with time, reaching a minimum after 24 hr. When the cells subjected to flow for 24 hr were returned to stationary state, the reduced VCAM-1 expression was almost completely restored in 72 hr, indicating that the change was reversible. The magnitude of the reduction of VCAM-1 expression was also dependent on the intensity of the wall shear stress applied, ranging from 0 to 7.2 dynes/cm2. These results, demonstrating an explicit down-regulating effect of flow on VCAM-1 expression of cultured ECs, suggested preferential adhesion of leukocytes to ECs at low shear regions at the vascular wall.

Close correlation between cytoplasmic Ca++ levels and release of an endothelium-derived relaxing factor from cultured endothelial cells

We studied whether there is a quantitative relationship between free cytosolic Ca++ levels and the release of an endothelium-derived relaxing factor (EDRF) from cultured fetal bovine aortic endothelial cells (EC). EC pretreated with indomethacin were stimulated by the agonists adenosine triphosphate (ATP), bradykinin (BKN), acetylcholine (ACh) and calcium ionophore (A23187) in various concentrations (10(-8)-10(-5) M), and the amount of EDRF released was determined on the basis of endothelium-free rabbit aortic ring relaxation and cultured smooth muscle cell cGMP content. Changes in intracellular Ca++ concentration ([Ca++]i) in response to the same stimuli were determined by photometric fluorescence microscopy using the fluorescent calcium indicator Fura-2. EC stimulation by ATP and A23187 induced dose-dependent increases in both [Ca++]i and the amount of EDRF released. BKN increased both [Ca++]i and EDRF release upon initial exposure (10(-8)M), but there were no further changes at higher concentrations. ACh induced no significant changes in either [Ca++]i or EDRF release. There was a close quantitative correlation between agonist-induced changes in [Ca++]i and the amount of EDRF released (relaxation response in aortic rings and cGMP levels.) (p < 0.001) Removal of extracellular Ca++ eliminated continuous elevation in both [Ca++]i and the amount of EDRF induced by ATP (10(-5)M), BKN (10(-8)M) and A23187 (10(-6)M). These findings suggest that intracellular Ca++ levels are directly linked to the amount of EDRF released, and that extracellular Ca++ is essential for EDRF release because its influx is involved in the continuous elevation of [Ca++]i.

Wall shear stress rather than shear rate regulates cytoplasmic Ca++ responses to flow in vascular endothelial cells

Recent evidence suggests that the vascular endothelial cell (EC) can sense the flow-rate over its surface and according to the information, regulates not only its own morphology and functions but also those of the surrounding smooth muscle and other tissues. There is now a discussion over which of the following mechanisms actually initiates the signal-transacting response of EC against flow: the mechanical shear deformation of the cell due to flow-oriented wall shear stress (tau), or the diffusional accumulation of vasoactive agonists on the cell surface modulated by wall shear rate (gamma) or both. To identify the relative importance of each mechanism, we examined quantitative changes in the cytoplasmic free Ca++ concentration ([Ca++]i) in cultured EC in the presence of the Ca++ mobilizing agonist ATP, i.e., a second messenger response of the internal signalling system, following the perfusion of two buffers with different viscosities (mu), which relates these factors as tau = mu gamma. The results of in vitro fluorescence photometry in EC with Fura-2 showed that the [Ca++]i level was enhanced with increase in the shear rate but to a greater extent with higher viscosity, and that the [Ca++]i levels at the same calculated level of shear stress were virtually identical, regardless of difference in shear rate and viscosity. This quantitative one-to-one relationship between the shear stress and the second messenger response suggests that wall shear stress rather than wall shear rate is the principal physical factor eliciting EC responses to flow.

Effect of extracellular ATP level on flow-induced Ca++ response in cultured vascular endothelial cells

Cultured vascular endothelial cells loaded with the highly fluorescent Ca(++)-sensitive dye Fura-2 were exposed to the flow of a fluid containing various concentrations of ATP (0, 0.5, 1, 5 microM) in an apparatus designed on the basis of fluid dynamics, and simultaneous changes in intracellular free Ca++ concentration were monitored by photometric fluorescence microscopy. The flow rate of the perfusate was altered from 0 to 6.3 to 22.8 to 39.0 cm/sec, inducing shear stress on the cell surface of 0, 2.9, 10.4, and 17.9 dynes/cm2, respectively. Although no significant change in intracellular Ca++ level was observed at ATP levels below 100 nM, at an ATP level of 500 nM, the intracellular Ca++ level increased together with an increase in the flow rate of the perfusate. At this level of ATP, the intracellular Ca++ levels at flow rates of 0, 6.3, 22.8, and 39.0 cm/sec were 44.8 +/- 7.3, 60.3 +/- 10.7, 74.0 +/- 5.8 and 89.4 +/- 6.4 nM (mean +/- SD; n = 8), respectively. At ATP levels over 1 microM, the flow-rate dependency of Ca++ response became less clear than that observed at the ATP level of 500 nM. These Ca++ responses to changes in flow rate disappeared when extracellular Ca++ was chelated by adding 2 mM of EGTA to the perfusate. These results suggest that the vascular endothelial cell has a mechanism that elevates the intracellular Ca++ level in accord with the flow rate at appropriate ATP concentrations, and that changes in intracellular Ca++ level under this mechanism seem to be chiefly caused by the influx of extracellular Ca++ into cells.